The preparation of mixtures of polyhydric alcohols, hydroxy aldehydes and hydroxy ketones by the auto-condensation of formaldehyde hydrate has been described in numerous literature references, for example the following: Butlerow and Loew, Annalen 120, 295 (1861); J. pr. Chem. 33, 321 (1886); Pfeil, chemische Berichte 84, 229 (1951); Pfeil and Schroth, chemische Berichte 85, 303 (1952); R. D. Partridge and A. H. Weiss, Carbohydrate Research 24, 29-44 (1972); the formoses of glyceraldehyde and dihydroxy-acetone according to Emil Fischer; German Pat. Nos. 822,385, 830,951 and 884,794, U.S. Pat. Nos. 2,224,910; 2,269,935 and 2,272,378 and British Pat. 513,708. These prior art processes have, however, certain disadvantages (toxicologically harmful catalysts, low volume/time yields and colored by-products). New processes have recently been developed by which virtually colorless formoses free from unwanted by-products may be obtained in high yields with the aid of the conventional catalysts.
According to one of these new processes, the condensation of formaldehyde hydrate is carried out in the presence of catalysts consisting of soluble or insoluble lead (II) salts or of lead (II) ions attached to a high molecular weight carrier and in the presence of a co-catalyst consisting of a mixture of hydroxy aldehydes and hydroxy ketones of the type obtained by the condensation of formaldehyde hydrate. The co-catalyst mixture is characterized by the following molar ratios:
Compounds having 3 carbon atoms/compounds having 4 carbon atoms: from 0.5:1 to 2.0:1; PA0 Compounds having 4 carbon atoms/compounds having 5 carbon atoms: from 0.2:1 to 2.0:1; PA0 Compounds having 5 carbon atoms/compounds having 6 carbon atoms: from 0.5:1 to 5.0:1;
wherein the proportion of components having from 3 to 6 carbon atoms is at least 75%, by weight, preferably more than 85%, by weight, based on the total quantity of cocatalyst.
The reaction temperature employed is generally from about 70.degree. to 110.degree. C., and preferably from 80.degree. to 100.degree. C. The pH of the reaction solution is adjusted by controlled addition of an inorganic or organic base so that it is maintained at from 6.0 to 8.0, preferably from 6.5 to 7.0 until from 10 to 60%, preferably from 30 to 50% conversion has been obtained. Thereafter, the pH is adjusted to a value of from 4.0 to 6.0, preferably from 5.0 to 6.0. It is surprisingly found that the ratio of components obtained in the resulting polyol, hydroxyaldehyde and hydroxy ketone mixtures may be varied in a reproducible manner by this particular method of pH control followed by cooling at different residual formaldehyde contents (from 0 to 10%, by weight, preferably from 0.5 to 6%, by weight).
When the auto-condensation of formaldehyde hydrate has been stopped by cooling and/or by inactivation of the lead catalyst with acids, the catalyst may be removed in known manner and the water contained in the products is evaporated off. Further details may be found in German Offenlegungsschrift No. 2,639,084.
According to another method by which highly concentrated colorless formoses may be prepared in high volume/time yields, aqueous formalin solutions and/or paraformaldehyde dispersions are condensed in the presence of a soluble or insoluble metal catalyst and in the presence of a co-catalyst. The co-catalyst is prepared by the partial oxidation of a dihydric or polyhydric alcohol (or mixture thereof) having a molecular weight of from 62 to 242 and at least two adjacent hydroxyl groups. During the condensation, the pH of the reaction solution is maintained at from 6.0 to 9.0 by controlled addition of a base until from 5 to 40% of the starting materials have undergone reaction. The reaction mixture pH is then adjusted to from 4.5 to 8.0 until the condensation reaction is stopped. In this second phase of the reaction the pH is from 1.0 to 2.0 units lower than in the first stage. The reaction is stopped by inactivation of the catalyst when the residual formaldehyde content is from 0 to 10%, by weight. The catalyst is then removed. This method has been described in detail in German Offenlegungsschrift 2,714,084.
Qualitatively superior formoses may also be prepared by the condensation of formaldehyde in the presence of a metal catalyst and more than 10%, by weight, based on the formaldehyde, of one or more dihydric or polyhydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds. Formose-polyol mixtures of this type are the subject matter of German Offenlegungsschrift No. 2,714,104.
It is particularly economic to prepare formose directly from formaldehyde-containing synthesis gases, i.e. without first obtaining aqueous formalin solutions or paraformaldehyde. For this purpose, the synthesis gases obtained from the large scale industrial production of formaldehyde are conducted continuously or discontinuously at temperatures of from 10.degree. to 150.degree. C. into an absorption liquid which consists of water, monohydric or polyhydric low molecular weight alcohols and/or higher molecular weight polyhydroxyl compounds and/or compounds capable of enediol formation as co-catalysts and/or soluble or insoluble metal compounds as catalysts, optionally attached to a high molecular weight carrier. The absorption liquid is at a pH of from 3 to 10. The formaldehyde is directly condensed in situ in the absorption liquid (or, if desired, in a reaction tube or a cascade of stirrer vessels situated behind the container for the absorption liquid). Auto-condensation of formaldehyde is stopped by cooling and/or inactivation of the catalyst using acids when the residual formaldehyde content in the mixture is from 0 to 10%, by weight, and the catalyst is finally removed. For further details about this process, see German Offenlegungsschrift No. 2,721,093.
Formoses prepared as described above may subsequently be converted into the corresponding hemiacetals with excess formaldehyde or .alpha.-methylolated by reaction with formaldehyde in the presence of bases. Modified formoses of this type have also been described in some detail in German Offenlegungsschrift No. 2,721,186.
The properties of formose (average hydroxyl functionality, degree of branching, number of reducing groups) may be varied within wide limits by suitably controlling the formaldehyde condensation reaction. The further the degree to which the condensation reaction is continued, i.e. the lower the residual formaldehyde content when the condensation reaction is stopped, the higher will generally be the average molecular weight and hence hydroxyl functionality of the formoses obtained. Thus, if the condensation reaction is continued to a residual formaldehyde content of from 0 to 1.5%, by weight, the resulting formose contains approximately 25%, by weight, of constituents having 5 carbon atoms, 45%, by weight, of compounds having 6 carbon atoms and approximately 20%, by weight, of compounds having 7 or more carbon atoms. At the same time, a total of only about 10% of polyols, hydroxy ketones and hydroxy aldehydes having 2, 3 or 4 carbon atoms is obtained. This corresponds to an average hydroxyl functionality of approximately 5.
If the formaldehyde auto-condensation is stopped at somewhat higher residual formaldehyde contents, different distributions of the components of the starting mixtures will be obtained, as mentioned above. When the condensation reaction is stopped at a formaldehyde content of from 2 to 2.5%, a mixture of polyhydric alcohols, hydroxy aldehydes and hydroxy ketones having an average hydroxyl functionality of approximately 4 is obtained. Yet other distributions of components having an even lower average hydroxyl functionality are obtained when the condensation reaction is stopped at residual formaldehyde contents even higher than 2.5.
The functionality of the products may be further varied as desired by mixing the formose with difunctional or higher functional low molecular weight alcohols if particular effects are desired for subsequent application of the products. Low molecular weight polyhydric alcohols (molecular weights up to about 300) which may be added for this purpose include, for example, ethylene glycol, propane-1,2-diol, propane-1,3-diol, butane-1,4-diol, diethyleneglycol, dipropyleneglycol, triethylene glycol, tetraethylene glycol, dibutylene glycol, glycerol, trimethylol propane, pentaerythritol, sorbitol, butane triols, hexane triols and the like and ethoxylation products of these alcohols, as well as hydrogenated formose (formite). Amines and/or ethanolamines may also be added to the mixture.
Examples of these include mono-, di- and tri-ethanolamine, mono-, di- and tri-isopropanolamine, N-alkanolamines, such as N-methyldiethanolamine and N-ethyldiethanolamine, and lower aliphatic monoamines and polyamines, such as ethylamine, ethylene diamine, diethylene triamine and triethylene tetramine.